Update nv_basic_test.cc (#24983)
### Description
Corrected dtype_name for the respective float16 implementations,
previously MLFloat16 would return bf16 rather than fp16, and vice-versa.
### Motivation and Context
It looked wrong but passed the tests, I don't fully comprehend what the
test suite is doing to try and improve it. I'd be willing to implement
any pointers.
